Abu Hurairah reported the prophet ﷺ as saying; when the imam recites “ not of those with whom thou art angry, nor of those who go astray” (surah al-fatihah,verse 7) say Amin, for if one’s words (utterance of amin) synchronise with those of the angles, he will be forgiven his past sins. (Using translation from Abū Dāʾūd 935)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا قَالَ الْإِمَامُ {غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ} [الفاتحة 7] فَقُولُوا آمِينَ فَإِنَّهُ مَنْ وَافَقَ قَوْلُهُ قَوْلَ الْمَلَائِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ